Method for the liquefaction of a hydrocarbon-rich fraction

ABSTRACT

A method for the liquefaction of a hydrocarbon-rich fraction is described, wherein the hydrocarbon-rich fraction is liquefied against an open expander refrigeration circuit. 
     According to the invention, that partial stream ( 6 - 14 ) of the hydrocarbon-rich fraction ( 3 ) to be liquefied which circulates in the expander refrigeration circuit is expanded in two stages so as to perform work (X 1 , X 2 ), wherein the partial stream ( 9 ) expanded in the first expansion stage (X 1 ) is cooled (E) and subsequently supplied ( 10 ) to the second expansion stage (X 2 ).

The invention relates to a method for the liquefaction of a hydrocarbon-rich fraction, wherein the hydrocarbon-rich fraction is liquefied against an open expander refrigeration circuit.

Generic methods for the liquefaction of hydrocarbon-rich fractions are used for example for the liquefaction of natural gas. An open expander refrigeration circuit is to be understood to mean a liquefaction process in which, before cooling and liquefaction, a partial stream of the (hydrocarbon-rich) charge fraction to be liquefied is extracted, cooled, expanded so as to impart a refrigeration action and subsequently heated in countercurrent with respect to the rest of the (hydrocarbon-rich) charge fraction to be liquefied. Open expander refrigeration circuits therefore do not require any additional or imported refrigerant or refrigerant mixture, for which reason the refrigerant (mixture) availability at the selected site of the liquefaction process is not of importance. Liquefaction plants in which such processes are realized are therefore highly flexible with regard to the selection of their site. Below, the expression “refrigerant” is to be understood to mean a single-component or multi-component refrigerant.

In the liquefaction of hydrocarbon-rich fractions, in particular of natural gas, use is made of open expander refrigeration circuits in which that partial stream of the charge fraction to be liquefied which is branched off as refrigerant is expanded, so as to impart a refrigeration action, in an expansion stage, preferably by means of an expander. By comparison with so-called closed expander refrigeration circuits and by comparison with mixture circuits, open expander refrigeration circuits have relatively poor energy efficiency. For this reason, in the past, they have normally been considered only for relatively small liquefaction plants—these being understood to mean plants with a liquefaction rate of a few hundred tonnes of LNG per day.

It is an object of the present invention to specify a generic method for the liquefaction of a hydrocarbon-rich fraction which exhibits better energy efficiency than liquefaction processes with open expander refrigeration circuits according to the prior art.

To achieve said object, there is proposed a method for the liquefaction of a hydrocarbon-rich fraction, said method being characterized in that that partial stream of the hydrocarbon-rich (charge) fraction to be liquefied which circulates in the open expander refrigeration circuit is expanded in two stages so as to perform work, wherein the partial stream expanded in the first expansion stage is cooled and subsequently supplied to the second expansion stage.

Further advantageous refinements of the method according to the invention for the liquefaction of a hydrocarbon-rich fraction, which constitute subject matters of the dependent patent claims, are characterized in that

-   -   that partial stream of the hydrocarbon-rich fraction to be         liquefied which circulates in the open expander refrigeration         circuit is additionally supplied to a third expansion stage,         wherein said third expansion stage is positioned upstream of the         above-described two-stage expansion,     -   the liquefied hydrocarbon-rich fraction is expanded and the         gaseous fraction formed during said expansion is separated off         and admixed again to the hydrocarbon-rich fraction to be         liquefied, with the gaseous fraction if appropriate being         compressed before the admixing, and     -   if the hydrocarbon-rich fraction to be liquefied and/or at least         one other process stream is subjected to a compression, at least         one of the expansion stages and the or at least one of the         compressions are realized in a compander.

According to the invention, that partial stream of the hydrocarbon-rich fraction to be liquefied which circulates in the open expander refrigeration circuit is now expanded in at least two stages. The efficiency of the liquefaction process can already be significantly improved by means of this procedure. A change from single-stage to two-stage expansion yields a reduction in energy consumption by approximately 20%. If a third expansion stage is additionally provided, the energy consumption is reduced by approximately 25% in relation to a single-stage expansion.

It is obvious that the outlay in terms of apparatus for the method according to the invention is greater than that for a liquefaction process in which an only single-stage work-performing expansion is performed. Said negative aspect is however compensated for by the advantages associated with the method according to the invention.

The method according to the invention for the liquefaction of a hydrocarbon-rich fraction, and further refinements of said method, will be explained in more detail below on the basis of the exemplary embodiments illustrated in FIGS. 1 and 2.

As illustrated in FIGS. 1 and 2, the hydrocarbon-rich charge fraction 1 to be liquefied is compressed to the desired liquefaction pressure by means of a single-stage or multi-stage compressor unit V1. The compressed charge fraction 2 is then split into two partial streams 3 and 6 or 6′, wherein only the partial stream 3 which constitutes the main stream is subjected to the cooling and liquefaction. For this purpose, said partial stream 3 is cooled in the heat exchanger E against process streams to be heated, which will be discussed in more detail below, and is at least partially liquefied.

The at least partially liquefied partial stream 4 is expanded in the valve a and supplied to a separator D. The liquefied hydrocarbon-rich product fraction 5 is extracted from the sump of said separator. The gaseous fraction 20 formed during the above-mentioned expansion in the separator D, said gaseous fraction being composed substantially of the components nitrogen and methane, is compressed in the compressor V3 to a pressure which permits subsequent admixing of said fraction to the refrigerant stream 12, which will be discussed in more detail below. The fraction 21 compressed in this way is subsequently conducted through the heat exchanger E in countercurrent with respect to the hydrocarbon-rich fraction 3 to be liquefied, and is subsequently admixed to the above-mentioned refrigerant stream 12.

In the case of the procedure illustrated in FIG. 1, the above-mentioned second partial stream 6 or 6′ of the compressed charge fraction 2, which partial stream forms the refrigerant stream, is supplied to the heat exchanger E and cooled in the latter against process streams to be heated. The refrigerant stream 8 cooled in this way is expanded in the first expansion stage X1 so as to impart a refrigeration action and is subsequently supplied via line 9 to the heat exchanger E again and cooled in the latter. The cooled refrigerant stream 10 is then supplied to the second expansion stage X2 and expanded therein again so as to impart a refrigeration action. The expanded refrigerant stream 11 is then heated in the heat exchanger E against the above-described process streams to be cooled, and is supplied via line 12 to the single-stage or multi-stage compressor unit V2. In the latter, the refrigerant stream is compressed to a pressure which permits admixing of the refrigerant via line 14 to the hydrocarbon-rich fraction 1 to be liquefied.

The embodiment of the method according to the invention illustrated in FIG. 2 differs from that illustrated in FIG. 1 in that that partial stream 6′ of the hydrocarbon-rich fraction 1 to be liquefied which forms the refrigerant is supplied to a third expansion stage X3. The refrigerant stream 7 expanded therein is subsequently supplied to the heat exchanger E. The further procedure of the refrigerant stream 7 is identical to that of FIG. 1.

The illustrated compressors and expansion stages may advantageously be combined with one another in a variety of ways in one or more so-called companders. In a compander, the compressor unit(s) and the expander unit(s) are connected by means of a common transmission on the same shaft. The outlay in terms of apparatus for the method according to the invention can thus be significantly reduced.

A further option is for a circuit compressor which has an external drive—for example an electric motor—to be provided, and for each of the expanders to drive a blower seated on the same shaft. 

1. Method for the liquefaction of a hydrocarbon-rich fraction, wherein the hydrocarbon-rich fraction is liquefied against an open expander refrigeration circuit, characterized in that that partial stream (6-14) of the hydrocarbon-rich fraction (3) to be liquefied which circulates in the expander refrigeration circuit is expanded in two stages so as to perform work (X1, X2), wherein the partial stream (9) expanded in the first expansion stage (X1) is cooled (E) and subsequently supplied (10) to the second expansion stage (X2).
 2. Method according to claim 1, characterized in that the partial stream (6) which circulates in the expander refrigeration circuit is additionally supplied to a third expansion stage (X3), wherein said third expansion stage is positioned upstream of the two-stage expansion (X1, X2).
 3. Method according to claim 1, characterized in that the liquefied hydrocarbon-rich fraction (4) is expanded (a) and the gaseous fraction formed during said expansion is separated off (D) and admixed again to the hydrocarbon-rich fraction (1) to be liquefied.
 4. Method according to claim 1, wherein the hydrocarbon-rich fraction to be liquefied and/or at least one other process stream is subjected to a compression, characterized in that at least one of the expansion stages (X1, X2, X3) and the or at least one of the compressions are realized in a compander. 